Modulation control

ABSTRACT

A modulator arrangement for modulating an optical signal using a quadrature phase shift key for use in an optical wavelength division multiplex (WDM) optical communications system comprising a laser ( 2 ) for producing an optical signal of a selected wavelength, which signal is split by a splitter ( 4 ), each part of said split signal being applied to a respective phase modulator ( 6, 8 ) each for which phase modulators is adapted to modulate the phase of the signal in dependence on a respective drive voltages. The phase of the output of at least one modulator ( 6 ) is shiftable at least in part by a phase shifter ( 10 ), the split signals being recombined by an optical recombiner ( 12 ) to form an optical phase shift key output, wherein the output power is monitored by a detector ( 20 ), the detector signal then being used to drive a feedback arrangement to control electrodes of the phase modulators ( 6, 8 ) and/or phase shifter ( 10 ).

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/GB02/05395, filed 29 Nov. 2002, whichclaims priority to Great Britain Patent Application No. 0128784.6 filedon 30 Nov. 2001, in Great Britain. The contents of the aforementionedapplications are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The invention relates to a method and apparatus for controlling aquadrature phase shift key encoder in a wavelength division multiplex(WDM) optical communications system.

In this specification the term “light” will be used in the sense that itis used generically in optical systems to mean not just visible lightbut also electromagnetic radiation having a wavelength between 800nanometres (nm) and 3000 nm. Currently the principal opticalcommunication wavelength bands are centred on 1300 nm, 1550 nm (C-Band)and 1590 nm (L-Band), with the latter bands receiving the majority ofattention for commercial exploitation.

Exemplary WDM systems operating in the 1550 nm C-Band optical fibrecommunication band are located in the infrared spectrum withInternational Telecommunication Union (ITU) 200, 100 or 50 GHz channelspacing (the so called ITU Grid) spread between 191 THz and 197 THz.

With ongoing developments in optically amplified dense wavelengthdivision multiplex (DWDM) optical links as the backbone ofpoint-to-point information transmission and the simultaneous increase inbit rate applied to each wavelength and the simultaneous increase in thenumber of channels, the finite width of the erbium gain window ofconventional erbium-doped optical amplifiers (EDFAs) could become asignificant obstacle to further increases in capacity. ConventionalEDFAs have a 35 nm gain bandwidth which corresponds to a spectral widthof 4.4 THz. System demonstrations of several Tbit/s data rate arealready a reality and the spectral efficiency, characterised by thevalue of bit/s/Hz transmitted, is becoming an important consideration.Currently, high-speed optical transmission mainly employs binaryamplitude keying, using either non-return-to-zero (NRZ) orreturn-to-zero (RZ) signing formats, in which data is transmitted in theform of optical pulses having a single symbol level, each symbolcorresponding to two bits.

In WDM several factors limit the minimum channel spacing for binaryamplitude signalling, and in practice spectral efficiency is limited to˜0.3 bit/s/Hz. Although increasing the per-channel bit rate tends toreduce system equipment, there are several problems that need to beovercome for transmission at bit rates above 10 Gbit/s; these being:

-   -   dispersion management of the optical fibre links, this becomes        increasingly difficult with increased bit rate;    -   Polarisation mode dispersion (PMD) in the optical fibre causes        increased signal degradation;    -   Realisation of electronic components for multiplexing,        de-multiplexing and modulator driving becomes increasingly        difficult.

One technique which has been proposed which allows an improvement ofspectral efficiency is the use of quadrature phase shift keying (QPSK)[S. Yamazaki and K. Emura, (1990) “Feasibility study on QPSK opticalheterodyne detection system”, J. Lightwave Technol., vol. 8, pp.1646–1653]. In optical QPSK the phase of light generated by atransmitter laser is modulated either using a single phase modulator(PM) driven by a four-level electrical signal to generate phase shiftsof 0, π/2, π or 3π/2 representative of the four data states, or usingtwo concatenated phase modulators which generate phase shifts of 0 orπ/2 and π or 3π/2 respectively. A particular disadvantage of QPSK isthat demodulation requires, at the demodulator, a local laser which isoptically phase-locked to the transmitter laser. Typically this requiresa carrier phase recovery system. For a WDM system a phase-locked laserwill be required for each wavelength channel. It further requiresadaptive polarisation control which, in conjunction with a phaserecovery system, represents a very high degree of complexity.Furthermore, systems that require a coherent local laser are sensitiveto cross-phase modulation (XPM) in the optical fibre induced by theoptical Kerr non-linearity, which severely restricts the application tohigh capacity DWDM transmission.

It has also been proposed to use differential binary phase shift keyingOBPSK) [M. Rohde et al (2000) “Robustness of DPSK direct detectiontransmission format in standard fibre WDM systems”, Electron. Lett.,vol. 36]. In DBPSK data is encoded in the form of phase transitions of 0or π in which the phase value depends upon the phase of the carrierduring the preceding symbol interval A Mach-Zehnder interferometer witha delay in one arm equal to the symbol interval is used to demodulatethe optical signal. Although DBPSK does not require a phase-locked laserat the receiver it does not provide any significant advantages comparedto conventional amplitude NRZ signalling.

U.S. Pat. No. 6,271,950 discloses a differential phase shift keyingoptical transmission system, comprising a laser to generate an opticalsignal a delay encoder to provide a different delay for each of M inputchannels and an M channel phase modulator which phase modulates theoptical carrier signal with each of the differently delayed M inputsignal channels to form a time division multiplexed (TDM) phasemodulated optical signal.

SUMMARY OF THE INVENTION

The present invention seeks to provide an improved control method andapparatus for an encoder for use in an optical phase shift key modulatorarrangement.

According to the invention there is provided a modulator arrangement formodulating an optical signal using a quadrature phase shift key for usein an optical wavelength division multiplex (WDM) optical communicationssystem comprising a laser for producing an optical signal of a selectedwavelength, which signal is split by a splitter, each part of said splitsignal being applied to a respective phase modulator, each of whichphase modulators is adapted to modulate the phase of the signal independence on a respective drive voltage, the phase of the output of atleast one modulator being shiftable at least in part by a phase shifter,the split signals being recombined by an optical recombiner to form anoptical phase shift key output, wherein the output power is monitored bya detector, the detector signal then being used to drive a feedbackarrangement to control electrodes of the phase modulators and/or phaseshifter.

The arrangement according to the invention advantageously controls theoperating point of the modulators and phase shifter of the quadraturePSK system by controlling the bias point of the modulator and theabsolute phase. This is useful both in the set up phase and during liveuse of the system.

Preferably the detector comprises a two photon absorption detectorPreferably, the feedback arrangement comprises an oscillator adapted toprovide a pilot frequency for the feedback arrangement, Preferably thefeedback arrangement comprises three control circuits, each driven at adifferent pilot frequency Preferably, the phase shift key is adifferential quadrature phase shift key. Preferably, the arrangementcomprises three two photon absorption detectors.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the invention will now be described ingreater detail with reference to the drawings in which

FIG. 1 shows an optical phase shift key modulator arrangement.

DESCRIPTION OF ILLUSTRATED EMBODIMENT

FIG. 1 shows an optical phase shift key modulator arrangement forencoding two 20 Gbit/s NRZ data streams U_(k), V_(k) onto a singleoptical carrier. Typically the modulator arrangement would be used aspart of a transmitter in a WDM optical communications system with arespective modulator arrangement for each WDM wavelength channel.

The modulator arrangement comprises a single frequency laser 2, forexample a distributed feedback (DFB) semiconductor laser due to itsstable optical output for a given wavelength, which is operated toproduce an unmodulated optical output of a selected wavelength,typically a WDM wavelength channel.

Light from the laser is transmitted to the integrated photonic device,where it is divided by an optical splitter 4 into two parts and eachpart is applied to a respective phase modulator 6, 8. Each phasemodulator 6, 8 is configured such that it selectively modulates thephase by 0 or π radians in dependence upon a respective binary (bipolar)NRZ drive voltage V_(I)(t), V_(Q)(t). In the preferred arrangementillustrated in FIG. 1 the phase modulators 6, 8 each comprise aMach-Zehnder electro-optic modulator (MZM). As is known MZMs are widelyused as optical intensity modulators and have an optical transmissionversus drive voltage characteristic, which is cyclic and is generallyraised cosine in nature. The half period of the MZM's characteristic,which is measured in terms of a drive voltage, is defined as V_(π).Within the modulator arrangement of the present invention each MZM 6, 8is required to operate as a phase modulator without substantiallyaffecting the amplitude (intensity) of the optical signal. To achievethis each MZM 6, 8 is biased for minimum optical transmission in theabsence of a drive, voltage and is driven with a respective drivevoltage V_(I)(t), V_(Q)(t)=±V_(π) to give abrupt phase shifting with aminimum of amplitude modulation. The two phase modulators 6, 8 havematched delays (phase characteristics).

The optical output from the phase modulator 6 is passed through a phaseshifter 10 which effectively applies a phase shift of π/2 such that therelative phase difference between the optical signals passing along thepath containing the modulator 6 and that passing along the pathcontaining the modulator 8 is ±π/2. The optical signals from the phaseshifter 10 and phase modulator 8 are recombined by an optical recombiner12, to form an optical phase shift key (PSK) output 14. In the case of aGaAs modulator arrangement, the splitter 4 comprises a 1×2 MMI(multimode interference coupler) and recombiner 12 comprises a 2×2 MMI.The two MMIs co-operate to provide a phase shift to the signal of aboutπ/2. A control electrode is then used to provide the fine control. Thereare of course alternative methods of obtaining a π/2 shift in one of thearms, such as using a control electrode to provide the entire shift.

To enable the power of the output signal to be monitored an output powerdetector 20 is provided at the combiner. The detector may comprise afast linear photodiode or a two photon absorber (TPA) detector. If themodulator is implemented in lithium niobate, then a linear photodiodetogether with an RF power detector is preferred and conversely in agallium arsenide based implementation a TPA detector is preferred as itfacilitates the production of a monolithic device. A particularadvantage of using a TPA detector is that only a small fraction of lightis absorbed and gives directly RF power detection obviating thenecessity for a separate splitter and photodetector, which would be usedshould the modulators be implemented using alternative materials such aslithium niobate. The TPA detector comprises an aluminium electrodeforming a Schottky contact over the waveguide at the combiner. Twophoton absorption is a non-resonant, non-linear optical process whichoccurs in semiconductor materials for photons having an energy less thanthe semiconductor band gap. The process occurs when an electron isexcited from the valence band to an intermediate virtual state in theband gap by absorbing a first photon and is excited to the conductionband by absorbing a second photon This generates a photocurrent which isrelated to the optical power in the waveguide. At the optical outputfrom the phase modulators 6, 8 are two two photon detectors (TPA) 60,70. The output of each TPA 60, 70 is fed to a separate control circuit50, 51, 52, 53, 54. Although it would be possible to rely solely on thepower detector 20 (TPA) for all three control circuits, a more precisecontrol will be obtained by the use of three detectors as betterdiscrimination of the individual pilot tones can be obtained since someinformation on the optical power may be lost when the signals arecombined.

The output of the detector is then fed to the control circuit. Aseparate control circuit is provided for each phase modulator 6, 8 andthe phase shifter 10. Each control circuit comprises a synchronousdetector and is functionally identical but operates at a differentfrequency. The use of the control circuit permits the optical phase tobe controlled to within +−2° (π/90 radians).

In an analogue implementation, it is envisaged that three separatecircuits, each operating at different frequency will be used. However,it would be possible to use a single digital circuit, which wasswitchable between the different frequencies.

In an implementation of the synchronous detector, the output of thedetector is fed to a multiplier 50, which is driven at a pilot frequencyprovided by a local oscillator 51. The output of the multiplier 50 isthen passed to an integrator 53 via a low pass filter 52. The low passfilter should have a cut off frequency lower than the difference infrequency between the adjacent local oscillators 51. The output of theintegrator is then fed to a summing amplifier 54 together with the localoscillator 51. The respective output of the summing amplifier 54 is thenfed back to the modulators 6, 8 and phase shifter 10, respectively.

The local oscillator 51 provides a pilot tone at a relatively lowfrequency well below the data bandwidth frequency. In a 20 GHz system,suitable frequencies for the three respective pilot tones would be ofthe order of 1 kHz, 2 kHz and 3 kHz. The dc output of the low passfilter acts to provide feedback to maintain the desired operating pointof the control loop. In an ideal system, the dc component would be zero.

The bias of the individual MZMs 6, 8 acts to adjust itself to obtain theminimum dc output from the respective low pass filter 52 of theircontrol circuit. Similarly the π/2 phase shifter acts to adjust itselfto obtain a minimum dc output from the low pass filter of its respectivecontrol circuit.

The phase modulator drive voltages V_(I)(t), V_(Q)(t) are generated bypre-coding circuitry 16 in dependence upon the two binary data streamsU_(k), V_(k). According to the modulator arrangement of the presentinvention the two data streams U_(k), V_(k) are differentially encodedsuch that these data are encoded onto the optical signal 14 in the phasetransitions (changes) rather than in the absolute phase value. As aresult it will be appreciated that the optical signal 14 is differentialquadrature phase shift key (DQPSK) encoded.

The DQPSK optical signal 14 is ideally given by E₀ exp(iωt+θ+θ_(i)),where ω is the mean optical angular frequency, t is time, θ the carrierphase and θ_(i) a data dependent phase modulation for the i-th datasymbol d_(i). In the general case d_(i)ε{0, 1, . . . M-1} and forquarternary phase shift keying M=4, that is the data symbol has fourvalues. The phase modulation term is given byθ_(i)=θ_(i−1)+Δθ_(i)(d_(i)) in which θ_(i−1) is the phase term for theprevious data symbol d_(i−1) and Δθ_(i) the change in phase between thei-₁ and i-th data symbols. The relationship between data symbol d_(i)and phase shift Δθ_(i) for QPSK is tabulated below.

TABLE 1 Values of data U_(k), V_(k), data symbol d_(i) and phase changeΔθ_(i) (d_(i)) for DQPSK. U_(k) V_(k) d_(i) Δθ_(i) (d_(i)) 0 0 0 0 0 1 1 π/2 1 0 2 π 1 1 3 3π/2

It is to be noted that the mapping between data, data symbol and phasechange is just one example and that other mappings can be used. Thepre-coding circuitry 16 is configured such as to produce the appropriatedrive voltages V_(I)(t), V_(Q)(t) in dependence upon the two datastreams d₁(t), d₂(t) according to the relationships:V _(I)(i)=V _(I)(i−I)cos Δθ(d _(i))−V _(Q)(i−I)sin Δθ(d _(i))  Eq. 1V _(Q)(i)=V _(I)(i−I)sin Δθ(d _(i))+V _(Q)(i−I)cos Δθ(d _(i))  Eq. 2.

1. A transmitter for modulating an optical signal using a quadraturephase shift key for use in an optical wavelength division multiplex(WDM) optical communications system, the transmitter comprising a laserfor producing an optical signal of a selected wavelength, said signal issplit by a splitter, each part of said split signal being applied to arespective phase modulator of the transmitter, each of said phasemodulators is adapted to modulate a phase of the signal in dependence ona respective drive voltage, the phase of the output of at least onemodulator being shiftable at least in part by a phase shifter of thetransmitter, the split signals being recombined by an optical recombinerto form an optical phase shift key output, wherein the output power ismonitored by a detector of the transmitter which generates a detectorsignal, the detector signal being adapted to drive a feedbackarrangement of the transmitter to control electrodes of one of the phasemodulator and the phase shifter of the transmitter.
 2. The transmitteraccording to claim 1, wherein the detector comprises a two photonabsorption detector.
 3. The transmitter according to claim 1, whereinthe feedback arrangement comprises an oscillator adapted to provide apilot frequency, said pilot frequency drives a multiplier and a summingamplifier of said feedback arrangement.
 4. The transmitter according toclaim 1, wherein the feedback arrangement comprises first, second andthird control circuits, each circuit being driven at a different pilotfrequency, the first and second circuits controlling the phasemodulators and the third circuit controlling the phase shifter.
 5. Thetransmitter according to claim 2, wherein the detector further comprisesthree two photon detectors, one detector being located on an output ofeach of the modulators and one detector being located on an output ofthe recombiner.
 6. The transmitter according to claim 1, wherein thequadrature phase shift key is a differential quadrature phase shift key.